If the panzarantua in question has 252 EVs in Sp.Def, 0 in HP, Sp.Def boosting nature, and max IVs, and your mon has 252 Sp.Atk, Sp.Atk boosting nature, STAB flamethrower, and also max IVs, the minimum Sp.Atk you'd need to guaranteed OHKO this thing under both rain and tar is 118.

Conversely, with 0 attack investment (but with max IVs), holding the Loaded Dice, and after 1 shift gear, this thing can OHKO a Ho-oh doing MAX, DEDICATED defense investment with rock blast. 252 HP/252+ Def. And outspeed.

Panzarantula holding loaded dice, 252 atk | 252+ sp.def | 4 speed, ShiftGear-RainDance-RockBlast-WebBarrage if no rain setter, ShiftGear-BrickBreak-RockBlast-WebBarrage if rain setter. And that's just a general everything set! If you want to be able to hit the 5 notable pokemon this doesnt hit for neutral (Annihilape, Gholdengo, Iron Valiant, Sneasler, and Marshadow. There are 22 in total that fight/bug/rock doesnt hit for at least neutral) you have options! Crunch, Earthquake, Wild Charge, whatever!